A low fuel consumption of a car has been attempted by reducing a rolling resistance of a tire (i.e. by improving fuel efficiency). Since a rolling resistance of a tire largely depends on heat build-up property of a rubber to be used for a tire member, so far development of a rubber for realizing low heat build-up thereof has been made actively, and various compounds being suitable for lower fuel consumption have been studied. Particularly as for a filler, not only a carbon black but also silica being advantageous for fuel efficiency has come to be used in many cases.
Meanwhile, so far a method of compounding a plurality of polymer (rubber) components (polymer blend) has been employed as a method for well-balanced improvement of various tire performances such as fuel efficiency, low-temperature property, performance on ice and snow and abrasion resistance. Specifically blending of some polymer components represented by a styrene butadiene rubber (SBR), a butadiene rubber (BR) and a natural rubber (NR) as rubber components of a tire has been mainly employed. This is a means for deriving physical properties of a vulcanized rubber composition by making use of characteristics of the polymer components while such properties cannot be derived only by a single polymer component.
In the polymer blend, a phase structure (morphology) of each rubber component and a distribution ratio of a filler such as silica in each rubber phase (a degree of localization of silica) after vulcanization will be important factors for determining physical properties. Elements for determining morphology and regulation of localization of a filler are very complicated, and various studies for exhibiting well-balanced physical properties of a tire have been made to date, and there is a room for improvement in any of the studies.
JP 2006-348222 A discloses a rubber composition for a side wall which is a polymer blend having a sea-island structure comprising a natural rubber and a butadiene rubber, wherein silica is localized in its discontinuous phase, however there is no description as to a polymer blend of styrene butadiene rubber and a butadiene rubber.